The present invention relates generally to mobile radiography, and more particularly, to means for improving the quality of images taken with mobile radiographic equipment.
In conventional hospital and nursing home practice, it is often necessary to take x-rays of patients who are confined to bed, for example, due to injury, or who are confined to the room, for example, due to contagious disease. In such instances, when the patient cannot be taken to a large fixed x-ray machine, mobile x-ray devices are brought to the patient. Radiographs taken at the bedside, however, are generally found to be inferior to those taken on the fixed apparatus in the x-ray department. In the latter, the equipment is fixed on tracks or the like so that alignment of the x-ray source, the patient and the cassette assembly can be precisely controlled. However, in the mobile x-ray situation, the x-ray source is carried on a movable trolley and the x-ray tube itself is essentially mounted on a universal joint so that it can be positioned in whatever attitude is needed to radiograph the desired portion of the anatomy of the bedridden patient. The film cassette, which carries the x-ray film, is then positioned on the side of the patient opposite to the x-ray source, and roughly in alignment with the source. It is typically held in position by whatever expedient is available; sliding it between the patient and the bed or pillow, propping it against the patient, and taping it to the rails of the bed are some examples. It will be appreciated that in that environment, alignment is usually less than precise.
In radiology in general, image sharpness and contrast of radiographs are seriously compromised by scattered radiation. This problem is controlled by the use of a grid. To deal with the scatter problem, this grid is interposed between the x-ray source and the film cassette. The grid can be a portion of a separate grid cassette into which the film cassette is inserted or alternatively installed independently but in front of the film cassette. Usually the grid is focused to the system focus-film distance, i.e., the distance between the focal spot of the x-ray source and the plane of the x-ray film. The purpose of the grid is to block scattered radiation from impinging on the film, allowing only radiation emanating directly from the focal spot to impinge on the film. The grid can be conceptualized as consisting of fine lead foil slats spaced at regular intervals in an arrangement analogous to a Venetian blind; the slats are focused at the focal distance, such as 40 inches, so that they are not perfectly parallel, but are "aimed" at the focal spot of the x-ray source. As a result, when using a grid, alignment becomes extremely significant. First of all, the proper focal distance must be maintained since the grid is focused on a spot which is a predetermined distance away. Secondly, alignment of the grid with respect to the central beam (i.e., the beam point which is normal to the grid) is essential, as can be readily appreciated when using a focused arrangement. Unfortunately in mobile radiography, such precise alignment is seldom achieved. When the grid is incorrectly positioned, "grid cut-off" results, i.e., the primary beam which should have impinged on the x-ray film is absorbed instead by the grid because it is improperly positioned.
As a consequence of these problems, many x-ray departments forego use of the grid in mobile radiography. The result is a consistently mediocre x-ray image. Those who use the grid, but align it by "eyeball technique", have the advantage of achieving improved images from time to time, but when misalignment occurs, which is often the case, achieving significantly inferior images.
The critical alignment necessary for use of a grid is achieved in fixed radiography by virtue of either a mechanical connection between the grid and the x-ray source, or an accurate light beam alignment system. However, in bedside radiography, where the x-ray tube is typically positioned at almost any angle in order to obtain a radiograph of a patient in bed, alignment of the grid becomes a significant problem.
Typically, an independent grid cassette is positioned beneath or next to the patient and the x-ray beam is directed at the patient in the general direction of the cassette. Unfortunately, placing the cassette between the patient and the patient support such as the pillow/bed makes it is very difficult to accurately align the grid cassette so that it is perpendicular to the x-ray beam.
To address the problem, some hospitals use a low ratio grid which requires less critical alignment, but which results in additional scatter impinging on the x-ray film which reduces the image quality. Even with a low ratio grid, if the grid is misaligned the image deteriorates. The conventional solution of removing the grid altogether prevents the obtaining of quality images because scattered radiation is not blocked from the film. When a grid is used, varying amounts of grid cut-off and variations in tube film distance cause the density of sequential radiographs to vary from one day to the next. This leads to difficulty in diagnosis. This grid cut-off effect often requires the repetition of the taking of the radiograph which brings about not only increased film consumption, and a waste of hospital personnel, but also an increased patient dosage of radiation.
In the x-ray department, accurate grid alignment and focal distance determination is achieved by mechanical and/or visual alignment devices. In mobile radiography, visual methods exist for focal distance determination but several problems make visual determinations difficult and impractical to use. Such devices do not solve the problem, but only serve as an aid for the technician. Since the grid cassette is normally placed between the patient and the bed, the grid cassette is largely obscured from sight by the patient's body, thus rendering the use of visual light beams very difficult. Because of this difficulty, beam alignment and focal distance determination is achieved by an "eyeballing technique" which inevitably varies the results. The distance can be measured with a tape, though in practice, it too, is often eyeballed.
Due to these limitations, the use of grids in bedside radiography is seriously limited. Because of these manual alignment problems, many hospitals will forego the grid entirely and accept the poor image quality.
At present, it is difficult to use mobile radiography to obtain either a horizontal beam for upright chest and abdominal exams or for decubitus views. Such views are difficult to obtain because the grid cassette must be propped or taped in position, again creating alignment problems. Alternatively, a separate mobile grid support device can be used, though this is not widely utilized for practical reasons. A horizontal view is highly desirable in radiography because when the patient stands or sits erect between the film and the x-ray device, air-fluid levels or abnormal air collections can be clearly demonstrated, though such findings might be invisible with a vertical or semi-vertical beam.